Phase modulation system



` 11s/VENTO MEM/rn -Ia "uw wr-'ramer PDPPDPP L. E. NORTON PHASEMoDuLA'r'IoN SYSTEMA Filed sept. 429, 1945 Feb. 22, 1949.

estema Feb. r2, 1949 2,462,417

UNITED STATES .PATENT OFFICE:

A 2,462,411 Y' f PHASE MonULA'rloN SYSTEM the source I.

Application September 29, 1945, Serial No. 919,400 z claims. (ci.ssa-24) This invention relates-to phase modulation sys- 'A tems. and itsprincipal object is to provide a sysand is much shorter than thewavelength corresponding to the highest modulation frequency to be used.I

A pair'of electron discharge tubes 1 and 9 are provided withtheir anodesconnected together-to a common load resistor II. Each or the tubes/2o 1and 'Sincludes two control grids as well as'the anode and cathode, andmay include further elements such as screen and suppressor grids. Thetubes l and 9 may be of the type designated in the radiov art as 6L7s.The tubes 'I and 9 are provided with self-bias resistors I3 and I5,bypassed by capacitors I1 and I9 respectively inv "accordance withconventional practice. v

Theinner control grid of the tube 1 is connected directly to the carriersource I. The inner control grid of the tube 9 is connected to theterminating'resistor-5, and' through the line 3 to The outer control'.grids are connected to opposite terminals of a` source 2| 'of modulationvoltage. The sourcevZI;4 is provided with a center-tapped outputcircuit,'with the center tap grounded, so asf to' excite the outer'control grids of the tubes l and 9 in push pull. The source 2| mayinclude a push pull output transformer, a phase inverter circuit, or anyother known means for providing balanced-to-ground output voltages.

The load resistor II is coupled to theinput of a voltage limiter 23. Theoutput circuit of the limiter 23 is coupled through a illter 25 to theutilization circuit, not shown. The illter 25 is designed to rejectharmonics of the carrier frequency, passing only the phase modulatedcarrier.

The operation of the described system is as i follows:

where E is the carrier amplitude. u is 2f times the carrier frequency, ais one-half the length oi the line 3. and A isthe wavelength at thecarrier frequency. The Initial phase angle is introduced merely forconvenience in the following analysis.

Since the line 3 is so shortl that its attenuation may be neglected,the'voitage across the line at i.

its midpoint, A, is:

8=E sin wt and the voltage across the terminating resistor 5 is:

`e1=E sin (wt ma?) The voltage at 'the inner control grid of the tubel.referred to the cathode, is

'.=E.|:1+m sin (ai where E1 is the bias between grid and cathode andwhere E is as above, the carrier amplitude. Similarly, the voltage atthe inner control grid of the tube 9, referred to its cathode, is:

modulation frequency. In the circuit as shown fin the, drawing, E2 andE1 are equal, but it will be apparent without further illustration thatthey may be made different by4 providing separate bias circuits for theinner and outer control grids. The

vvoltage at the anode of the tube 1 is:

e.=kE.E,[1+m sin (wwQ-rm, sin a+ I mm vsinxt sin (wt+g;a)] j =kE1E2[1-lmsin wt cos glie-rm cos wt sin 2%a+ m3' sin t-I-mmg sin Bt sin wt singil-a+ mm2 sin 4t cos wt sin -2-{2} Similarly, the voltage at the outercontrol grid of the t'gube 9, referred to its cathode, is:

e2'=E2(1-m2 Sin t) and the voltage at the anode of the tube 9 is:

'e3' =kE1E '2[1-im sin wt cos misin )St-mm2 sin pt sin at cos Il-2+ mn,sin Bt cos wt sin ir-q] K The voltages es and es are' combined toproduce a voltage e4 on commondoad impedance I I:

) cos wt] This may be rewritten as:

i sin [ast-i-tau-1 (mz tan z q sin 135)] mm2 sin Bt sin In the aboveexpression, m2 lies between 0 and 1, 1

i. e. the amplitude of-the modulating voltage is no greater than thebias voltage En on the outer control grids, and l sin tl Since thelength 2a of the line 3 is short compared to A, as stated above,

21rd. 21rd and 21rd tan (m2 tan T sm 5t is substantially equal to:

is some constant phase displacement, Accordingly, e4' may be written as:

represents amplitude modulation. lThis is removed by the limiter 23. Thelimiter 23, however, acts to introduce various harmonics of the carrierfrequency cycles per second.

It will be apparent without further illustration that the describedsystem will operate with complex modulation, such as that derived fromspeech or music, in the same` manner as with simple sinusoidalmodulation. The modulation factor mz is limited only by the maximumsignal amplitude which can be applied to the grids of the tubes 1 and 9,and the length of the delay line 3.

Thus I have provided an improved phase modulator system including a pairof modulator tubes with their output circuits in parallel, and excitedin push pull by the modulation voltage. Carrier voltage is applied toone tube directly and with a xed delay to the other tube.` This providesan output whi-ch is both phase modulated and amplitude modulated. Theamplitude modulation is removed by a limiter, providing a pure linearlyphase modulated output.

I claim as my invention: f

1. A phase modulation system including a, cart rier source of nominallyconstant frequency but subject to undesired variations, a pair ofelectron discharge tubes each having at least an anode, a cathode andtwo control grids, means for applying the output of, said carrier sourcedirectly to the iirst control grid of the rst of said tubes, and meansapplyingthe output of said carrier source with a delay time which issubstantially independent of said variations in frequency of saidcarrier source to the corresponding control grid of the other of saidtubes, a source of modulation voltage, means for applying saidmoduletion voltage to the second control grids of both of said tubes inpush pull, a common load impedance connected to the anodes of both oisaid tubes in parallel, a voltage limiter connected to said loadimpedance, and lter means connected t-o the output circuit of saidvoltage limiter.

2. A phase modulation system including a carrier source of nominallyconstant frequency but subject to undesired variations, a pair ofelectron discharge tubes each having at least an anode, a cathode andtwo control grids, means for applying the output of said carrier sourcedirectly to the first control grid of the first of said tubes, and meansapplying the output of said carrier source with a delay time which issubstantially independent of said variations in frequency of saidcarrier source to the corresponding control grid of`the other ofsaidtubes, a source of modulation voltage, means for applying saidmodulation voltage to the Asecond control grids f both of said tubes inpush pull, and a common load circuit connected to the anodes of both ofsaid tubes in parallel.

' LOWELL E. NORTON.

(B eferences on following page) 5 m mm) Number B ons 2,238,249 Thefollowing references are of record ln th 2,383,093 111e of this patent:

UNITED STATES PATENTS 5 Number Number Name Date 35,902

2,165,229 Crosby. July 11, 1939 2,210,015 Usselman Aug. 6, 1940 6 NameDate Crosby Apr. 15, 1941 Usselman Oct. 30, 1945 FOREIGN PATENTS CountryDate Netherlands July 15, 1935

